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Structure and Dynamics of Solar Atmosphere

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Updated July 31, 2009

Fine structure of sunspots has been studied using observations obtained with the 1-m Swedish Solar Telescope (SST) at the Observatorio del Roque de los Muchachos, La Palma, and with the Solar Optical telescope (SOT) onboard the HINODE satellite.

In collaboration with IAC, kinematics of umbral fine structures was studied in a series of broad-band images acquired at SST. For this purpose, a newly improved feature-tracking code was applied. The small-scale structures move with average speeds of 0.34 km/s either into the umbra or along faint light bridges. Structures that do not split or merge during their life are smaller (0.15") than the average size (0.17") of all features. Brightness and size variations of individual non-split/merge structures are positively correlated during their evolution.

In collaboration with NAOJ, high cadence observations taken in 2006 with the Hinode satellite through the Ca II H and G-band filters were analysed to determine the inclination of penumbral microjets. The results were then compared with the inclination of the magnetic field determined through the inversion of the spectropolarimetric observations of the same region. The penumbral microjet inclination is increasing towards the outer edge of the penumbra. The results suggest that the penumbral microjet follows the opening magnetic field lines of a vertical flux tube that creates the sunspot.

A time series of full-Stokes spectropolarimetric observations of a sunspot acquired with HINODE/SOT in 2007, was inverted using the code SIR into a series of 34 maps covering 3 hours of umbra and penumbra evolution. The retrieved maps show the spatial distribution of temperature, line-of-sight velocity, magnetic field strength and inclination in the low and high photosphere. In these maps, the evolution of central and peripheral umbral dots and penumbral grains was traced. While central umbral dots do not show any excess of line-of-sight velocity and magnetic field inclination with respect to the surrounding umbra, upflows and a more horizontal magnetic field are detected in low photospheric layers of peripheral umbral dots. Penumbral grains have even stronger upflows and magnetic field inclination in the low photosphere than peripheral umbral dots. Absolute values of these parameters decrease when penumbral grains evolve into peripheral umbral dots. It seems that penumbral grains and peripheral umbral dots are of a similar physical nature. Both classes of features appear in regions with a weaker and more horizontal magnetic field and their formation height reaches the low photosphere. On the other hand, central umbral dots appear in regions with stronger and more vertical magnetic field and they are formed too deep to detect upflows and changes in magnetic field inclination.

Large-scale flows in the solar photosphere were studied in collaboration with the Stanford University, using the helioseismic observations and the local correlation tracking method applied to full-disc Dopplergrams acquired by the Michelson Doppler Imager (MDI) onboard SOHO. Effects of solar active regions on meridional flows were analysed to extend the previous study of the solar-cycle variations of meridional flows and to investigate their latitudinal and longitudinal structure in the subphotospheric layers, especially their variations in magnetic regions. Helioseismology observations indicate that mass flows around active regions are dominated by inflows into those regions. On average, those local flows are more important around the leading magnetic polarities of active regions than around the following polarities and depend on the evolutionary stage of particular active regions. A statistical study based on MDI/SOHO observations of 1996?2002 shows that this effect explains a significant part of the cyclic change of meridional flows in near-equatorial regions, but not at higher latitudes. A different mechanism driving solar-cycle variations of the meridional flow probably operates.

Since we did not precisely know whether the surface flow fields derived from the local correlation tracking of supergranules represent the flow field in the photosphere or in some subphotospheric layers, we combined them with helioseismic data and studied the vertical structure of large-scale horizontal flows. We were able to estimate the depths in the solar convection envelope, where the detected large-scale flow field was well represented by the surface measurements. We got a clear answer to the question what kind of structures we tracked in full-disc Dopplergrams. It seems that in the quiet Sun regions the supergranular structures are tracked, while in the regions with the magnetic field the structures of the magnetic field are dominant. This observation seems obvious, because the nature of Doppler structures is different in the magnetic regions and in the quiet Sun. We showed that the large-scale flow detected by our method represents the motion of plasma in layers down to 10 Mm. The supergranules may therefore be treated as the objects carried by the underlying large-scale velocity field.

The 3D coronal structures and magnetic field during the total solar eclipse were studied. The good quality of the observing sequence of about 60 photographs of the white-light corona taken during the total solar eclipse observations on 29 March 2006, in Al Sallum, Egypt, enabled us to use a new method of image processing for enhancement of the fine structure of coronal phenomena. We presented selected magnetic field lines derived for different parameters of the extrapolation model. The coincidence of the observed coronal white-light fine structures and the computed field-line positions provides a 3D causal relationship between coronal structures and the coronal magnetic field.

Updated November 20, 2007

Fine structure of sunspots and pores has been studied using numerous observations obtained with the old 0.5-m Swedish Vacuum Solar Telescope (SVST), with the new 1-m Swedish Solar Telescope (SST) at the Observatorio del Roque de los Muchachos, La Palma, and with the Vacuum Tower Telescope (VTT) at the Observatorio del Teide, Tenerife. In collaboration with Instituto de Astrofisica de Canarias, an analysis of high-resolution 2D full-Stokes observations of light bridges and inner penumbra has been performed. The observations were recorded with the La Palma Stokes Polarimeter attached to the SVST. The stratification over the solar atmosphere of different physical parameters was retrieved using the SIR inversion code, thus obtaining semi-empirical models of the thermal structure, magnetic and velocity fields. The models, computed for each position in the field of view, were used to create 3D maps of temperature and magnetic field vector. A canopy structure above the light bridges was deduced from these maps. Light bridges are formed by deeply rooted non-magnetic or weakly magnetic regions in the umbra, where a convection takes place. In the inner penumbra, our results suggest the existence of unresolved magnetic and temperature structures. The stratifications of the temperature and of the magnetic field strength indicate the presence of rising flux tubes, predicted by theoretical simulations. High-resolution 2D spectropolarimetric observations acquired with the Fabry-Perot interferometer attached to the VTT were used to analyze the magnetic field and flow topologies of a penumbral connection between two opposite-polarity solar pores. The obtained results are in accordance with the widely-accepted uncombed penumbra hypothesis and the rising flux tube model. In collaboration with Institut für Astrophysik of the Göttingen University, a time series of white-light images acquired at the SST in 2004 was analyzed, showing the faintest umbral fine structures. In addition to umbral dots, often clustered to more stable "granules", large, low-intensity elongated structures with dark central channels were observed. At the periphery of the umbra, bright umbral dots move inwards, showing a similarity to penumbral grains. Kinematic properties of umbral fine were also studied.

An analysis of horizontal motions in sunspot moats was made in collaboration with Observatoire Midi-Pyrénées. Applying the local correlation tracking technique to series of white-light and ultraviolet images acquired by the satellite TRACE, motions of granules and C IV emission features were used to determine areas, horizontal velocities, and asymmetries of moats around 32 sunspots of different sizes, shapes, and phases of evolution. Observed asymmetries in moat shapes and velocities are related to the height in the atmosphere, sunspot age and proper motion. It is suggested that the sub-photospheric convective flows around sunspots may be influenced by the spots? proper motion through the convection zone.

Large-scale flows in the solar photosphere are studied using the local correlation tracking method applied to full-disc dopplergrams acquired by the Michelson Doppler Imager (MDI) onboard SOHO. For our studies, the high cadence data are used, covering approximately two months continously each year since SOHO was launched. Until now we have processed nearly 3 TB of primary data. The data undergo sophisticated processing in order to suppress noise and other disturbing effects. The method was developed, tested and tuned using synthetic data provided by a simple simulation. We have found that the method is capable to measure surface velocities with an accuracy of 10 m/s and that its results are consistent with the measurements made by local helioseismology. The method clearly reveals widely accepted properties of intergral large-scale flows, such as differential rotation, meridional circulation and a pattern of torsional oscillations. The long-term dataset shows a strong influence of the magnetic activity in the equatorial belt on solar equatorial rotation. In the period of strong magnetic activity, the equatorial rotation is generally slower, although local magnetic fields slightly speed-up the motions in their vicinity. The application of the method to the data in the area of an erupting filament showed a clear increase of the shear in zonal velocity at the position of filament eruption in the pre-erupting phase and its sudden decrease shortly after the filament eruption.

A recent comparison of large-scale velocities inferred from horizontal transport of the magnetic flux, measured on WSO of Stanford University, Kitt Peak NSO and SOHO/MDI, with velocity field below the photosphere, derived by local helioseismology method from MDI observations, show a strong dependence of the velocity patterns on the spatial resolution of used measurements. In fact, the main velocity structure relates with magnetic flux on the small scale and the large-scale velocity structures are usually representative only for the long-time scale. The agreement between both methods is not permanent for the whole available data set and varies according to the stability of active regions occurrence in some extended regions on the Sun.

The total solar eclipse observations from Angola 2001 and Egypt 2006 were processed and combined using an iterative technique with a current-free extrapolation of magnetic field from the photosphere. The structures observed in white-light corona show a very good agreement with the extrapolated field-line structures and the calculated intensity values can be used also for the description of some physical conditions in the corona.

Updated September 23, 2005

The dynamics of and the relations between small-scale penumbral and photospheric features near the outer penumbral boundary have been investigated. A 2-hour time sequence of a decaying sunspot, taken at the Swedish Vacuum Solar Telescope (La Palma) and restored by the phase-diversity method was analyzed with the following results: One third of the outward-moving penumbral grains pass through the outer penumbral boundary and then either continue moving as small bright features or expand and develop into granules. G-band bright points observed next to the spot have not been identified as a continuation of penumbral grains escaping from the penumbra. They are mostly born close to dark penumbral fibrils where the magnetic field is strong, whereas penumbral grains stem from the less-magnetized penumbral component. The same data were employed to study the motions of granules and G-band bright points in the sunspot's moat. Centers of diverging horizontal motions, identified with families of granules formed by recurrently splitting granules, move away from the sunspot. Most of G-band bright points show motions in the same direction, with velocities comparable to those of adjacent granules.

The effect of Joule heating mechanism on the brightness of umbral dots during the phase of their decay has been investigated. Time series of high-resolution images of umbral dots in a developed sunspot and in a large pore were analyzed. The effect of Joule heating is characterized by a specific shape of temporal variations of brightness and area. In our observations, about 12 % of umbral dots in the sunspot and about 14 % in the pore show indications for a presence of the Joule heating.

An analysis of high-resolution full Stokes observations of eight umbral dots in a sunspot was made. The 2D spectra were recorded with the La Palma Stokes Polarimeter, attached to the Swedish Vacuum Solar Telescope. The observed line profiles have been inverted to yield the height stratifications of temperature, magnetic field, and line-of-sight velocity. Small upflows, higher temperatures, and weaker fields with more horizontal orientations with respect to the surrounding umbra were found. The observational signatures revealed by our analysis fit well within both the "spaghetti" and the monolithic models of sunspot umbrae. The full Stokes 2D spectra were also utilized to map the line-of-sight velocities and magnetic field strength and inclination in sunspot umbra, penumbra, and light bridges, using the Stokes inversion code (SIR) and complementary measurements of line shifts and bisectors in I-profiles. Velocity anomalies were observed at the interstices of one light bridge with the penumbra.

Observations of two sunspots and two pores, acquired in September 2003 with the new 1-m Swedish Solar Telescope, La Palma, have been analyzed. White-light images with resolution better than 0.15" were taken simultaneously in blue and red bands. Intensities and diameters of umbral dots were measured in pairs of aligned blue and red images. The "true" intensities and diameters were calculated using the method of two-color photometry. Histograms of observed diameters have maxima at 0.23" (165 km). This indicates that most of umbral dots can be spatially resolved with the 1-m telescope. The results of two-color photometry show that about 50 % of umbral dots are brighter and hotter than the mean quiet photosphere and that the average "true" diameter of umbral dots is 100 km.

Line-of-sight velocity and magnetic field components measured with the Ondrejov magnetograph in seven active regions have been analyzed. The velocities in active regions show specific disturbances of the semi-regular motion pattern typical for the quiet photosphere: In the most active phases of evolution, velocities away from the observer are concentrated and organized into cellular-like structures covering the magnetic areas, while velocities toward the observer in the same areas are almost diluted. In decaying active regions, the cellular structure of velocities disappears. The Evershed effect connected with disappearing sunspots was found to have the direction opposite to that in stable spots. The organized cellular structures of velocities are best visible near the disk center. This indicates a presence of steady downflows in developing active regions.

Series of the SOHO/MDI dopplergrams were used to study large-scale horizontal motions of supergranular structures. An original methodics and software has been elaborated for this purpose. The resulting global horizontal flows, obtained for the period of solar minimum, have dominant zonal and weaker meridional components and are consistent with time-dependent differential rotation of the Sun.

A dynamical theory of tidal waves on the solar surface has been elaborated in cooperation with the St. Petersburg State Technical University. It was utilized for modeling Doppler and horizontal velocity modulations caused by tidal waves induced by the planets Mercury, Venus, Earth, and Jupiter along the solar equator. Short periods of zero modulation alternate regularly with long periods of strong modulation every 120 days. The comparison with velocities derived from the SOHO/MDI dopplergrams has shown that the velocity amplitudes of tidal waves must be smaller than ? 20 m/s.

Large-scale horizontal velocities transporting the large-scale magnetic flux in the photosphere have been studied in terms of their variability with the 11-year solar cycle. They show clear variations of the equatorial zonal velocity and activity zone zonal velocity relative to the activity cycle. Similar variations were also found for meridional transporting velocities and for cyclic changes of the N-S symmetry of the flows. Residual zonal velocities of magnetic flux transport show equally distributed fast velocity patterns like those derived from Doppler measurements by other authors. All variations are determined by non-axially symmetric processes on the Sun. The increase of the zonal velocity on the account of the meridional velocity around the activity minima is well corresponding with prediction of the models of the solar cyclic dynamo action.

The large-scale dynamical characteristics of the solar corona, photosphere, and chromosphere were studied. For the solar corona, a magneto-dynamic model of coronal magnetic structures was constructed to transform current-free fields into force-free fields due to the large-scale horizontal velocities transporting magnetic flux in the photosphere. A test of the relationship between the closed magnetic field configurations in coronal space and extrapolated total magnetic fields in the low corona shows a close correspondence. Regions with enhanced values of the total magnetic field are responsible for enhanced intensity of the green corona mainly due to a local increase of the density of heated coronal structures. A complicated system of horizontal velocities transporting the magnetic flux regions in the photosphere is a source of continuous changes in the connectivity between opposite polarities. So, total magnetic flux regions and also the regions of the enhanced green corona brightness co-rotate and display apparently more rigid rotation than bipolar magnetic regions, filaments and sunspots.

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